Abstract

Reactive halogen species (i.e., RHS=X, XO, X2, XY, OXO, HOX, XONO2, XNO2, with X,Y being I, Br and Cl) are known to be key compounds for the oxidation capacity of the troposphere, affecting the lifetime of relevant species such as O3, HOx, NOx, hydrocarbons, dimethylsulfide and gaseous elementary mercury. Furthermore, recent observations link iodine species to the formation of new aerosol particles. This work aims at the characterization of the abundance of BrO in the Arctic troposphere during the spring season, when the auto-catalytic release of bromine species from sea ice related halides is known to cause tropospheric Ozone Depletion Events (ODEs). A novel limb scanning mini-DOAS spectrometer for the detection of UV/vis absorbing radicals (e.g., O3, BrO, IO) was deployed on the DLR-Falcon (Deutsches Zentrum für Luft- und Raumfahrt) aircraft and tested during a field campaign that took place at Svalbard (78°N) in spring 2007. Herein, a new algorithm for inferring concentration vertical profiles of tropospheric trace gases from aircraft-borne DOAS limb observations is presented, characterized and validated through the profile retrieval of the UV-A absorber O4. The method is then applied for retrieving tropospheric vertical profiles of BrO during the polar campaign. For deployments during ODEs, the retrieved BrO vertical profiles consistently indicate high BrO mixing ratios (~ 15 pptv) within the boundary layer, low BrO mixing ratios (< 1.5 pptv) in the free troposphere, occasionally enhanced BrO mixing ratios in the upper troposphere (~ 1.5 pptv), and increasing BrO mixing ratios with altitude in the lowermost stratosphere. These findings are well in agreement with satellite and balloon-borne soundings of total and partial BrO atmospheric column densities. Moreover, the capabilities of the aircraft-borne measurements are further exploited by analyzing the sources, photochemistry and transport processes of BrO in the boundary layer above the sea ice.